The mammalian neocortex is instrumental in a variety of cognitive functions, and understanding cortical circuitry is essential for prevention and treatment of mental disorders. A prominent feature of the cortex is that neurons with similar functional properties are organized in columns. However, how the columnar structure arises during development remains poorly understood. Our recent study in mouse visual cortex has shown that sister neurons originating from a common progenitor cell during early brain development exhibit strong similarity in orientation tuning, arguably the most important functional property in the primary visual cortex. This finding demonstrated, for the first time, a direct correspondence between the ontogenetic and functional columns, and it opened new avenues for studying cortical circuitry. The proposed study aims to address several basic questions concerning the developmental origin of cortical microcircuits, using a combination of techniques including two-photon imaging, electrophysiological recording, neuronal labeling, and molecular perturbation with recombinant viral vectors. By testing whether and how the functional similarity between sister neurons depends on their lineage and physical distances, we will define the basic processing unit of the cortex (Aim 1). By manipulating the excitability and NMDA receptor expression in selected neurons and visual experience of the animal, we will elucidate the interactions between developmental lineage, electrical activity, and visual experience in shaping the functional properties of cortical neurons (Aims 2 and 3). Finally, using high-speed circuit mapping with glutamate uncaging, we will detect common inputs to clonally related sister neurons in order to understand the synaptic basis for their functional similarity. These experiments represent a major step to bridge developmental and systems neuroscience, and they are likely to provide unprecedented insights into the development and organizational principle of cortical microcircuits.